Uptime Kuma is a self-hosted monitoring tool like "Uptime Robot". Installing it on a raspberry pi can easily be done by installing it using docker.
Browse to http://localhost:3001 after starting.
docker volume create uptime-kuma
docker run -d --restart=always -p 3001:3001 -v uptime-kuma:/app/data --name uptime-kuma louislam/uptime-kuma:1